The server role is a fixed role at the server level. Its permissions cannot be changed and an other server role can not bed added.
The server role gives the administrator access to SSAS.
Members of this role have access to all SSAS databases and objects on that instance of SSAS.
To give lower-level administrative rights, you should consider adding users to a database role and granting the relevant permissions to that.
Use SQL Server Management Studio to add members to the server role. You can add members to the server role on the Security page of the Analysis Server Properties dialog box.